Line cameras are generally used for the optical scanning of a document in a scanner. The line camera and the document are moved relative to one another, so that the surface of the document to be scanned is covered completely by the line camera.
Such line cameras are also often used in industrial applications to scan along predetermined paths of products being made. Often this involves automatic quality assurance with the aid of the images taken by the camera. These products may be e.g. paper printed by high-speed printers, or other machine-made web-like products.
In the starting and stopping of such production webs, they are slowly accelerated or decelerated. Proper monitoring of the products should also be possible during the acceleration and deceleration phases.
In conventional methods, an encoder is used to generate a clock signal proportional to the speed of the web, and the line frequency of the line camera is controlled on the basis of this signal.
The line cameras have charge coupled devices (CCDs), the exposure time of which can generally not be actively controlled. The exposure time of the CCD therefore corresponds to the time interval between two scans, less a predetermined duration of time required to read out the CCD. As a result of this, with lower line frequency, the exposure time of the CCD increases, with a consequent and corresponding increase in image brightness. At low line frequencies this can also lead to the CCD becoming completely saturated, so that image generation is no longer possible. A method of this kind permits only limited variations in speed.
Also known are CCDs with electronic shutters on the individual sensor elements. With these, the exposure time may be controlled actively, and the speed-related differences in brightness can be avoided. Such CCDs with electronic shutters are however very sluggish, and for a high-speed line camera they are not available.
In order to reduce these problems, a completely new approach to the optical scanning of a document has been proposed in DE 10 2004 050 422 A1 (see also, US 2006/0232702 A1). Here the camera is operated at a line frequency which is independent of the relative speed of movement between the camera and the document. The relative speed of movement between the camera and the document is recorded and, depending on this movement, the camera signals are completely scanned afresh, in order to generate an output image with a predetermined output resolution. In the course of this rescanning it is necessary to interpolate the camera signals, so that on the one hand some information is lost, while on the other hand the sharpness of the image is impaired.
A further development of this known method is described in DE 10 2005 013 850 A1 (see also US 2005/0212970 A1). Here the raw signals from the camera are converted by one or more analog/digital converters, and rescanned in accordance with the method described above. Apart from the reduction in quality of the output image, a disadvantage is that considerable computing power is required to implement this method, so that a computer or other logic circuit (e.g. field programmable gate array (FPGA)) with a suitably high capacity must be provided.
JP 62-55777 A discloses a method for the optical scanning of a document using a camera. During scanning, the document is moved at a speed of movement which is detected by a rotary coder. The line frequency of the line camera is set so that the resolution of the transmitted image remains the same even at varying speeds. Since image brightness is proportional to the integration time of the sensor, it is in inverse proportion to the speed. Fluctuations in speed may therefore be regarded as fluctuations in image brightness.
JP 6-38028 A shows a scanner which is able to scan a document at various reduced sizes. In this case, the line frequency of the image sensor is held constant, and the speed is varied according to the scale of reduction or enlargement. The smaller the chosen scale, the greater the speed that must be set. For a very large reduction, the color values of two adjacent lines may be added together, in which case these two lines are output as a single line. This further reduces the number of lines and the resolution, so that the actual scanning speed may be further reduced.